(1) Field of the Invention
The present invention relates to a honeycomb structural body especially used as a catalyst carrier in an exhaust gas purifying apparatus or a firing apparatus such as a boiler or a combustion apparatus such as an internal combustion engine.
(2) Related Art Statement
As the catalyst carrier of the exhaust gas purifying apparatus, the honeycomb structural body has been used. Generally, just after the automobile engine starts, i.e. in a so-called "cold start" state, in which a temperature of the exhaust gas from an exhaust gas generation source is low, it is necessary to increase a temperature ascending rate of the catalyst in the exhaust gas purifying apparatus to activate the exhaust gas purifying function as soon as possible. In order to achieve this requirement, a technique, whereby a thermal capacity of the catalyst is decreased (and thus a temperature ascending rate is made faster) by forming a partition wall of the honeycomb structural body thinner to an order of 0.15 mm or 0.1 mm, is disclosed in Japanese Patent Laid-Open Publication No. JP-A-7-39761.
Moreover, in order to further improve the exhaust gas purifying performance, a technique, whereby a geometric surface area of the honeycomb structural body is increased, i.e. the number of passages per unit area on a surface thereof perpendicular to a passage extending direction (hereinafter, sometimes called cell density) is increased, is disclosed in papers of No. 960560 and No. 960261 of SAE (Society of Automotive Engineers) in the United States.
With the above mentioned prior art, however, the following result is found. That is, in order to obtain an excellent exhaust gas purifying performance of the catalyst carrier including a ceramic honeycomb structural body, if the partition wall is formed extremely thin, i.e., to an order of smaller than 0.1 mm, and if the cell density is increased to an order of larger than 100 cells per cm.sup.2, a spalling strength of the honeycomb structural body indicating a thermal shock resistivity is largely decreased only when a ratio of L/d, wherein d is a diameter of the honeycomb structural body and L is a length thereof, exists in a certain range.
The spalling strength mentioned above is estimated as follows. At first, the honeycomb structural body is heated by an electric furnace or a burner using combustion gas and then cooled. Then, whether or not a defect of the honeycomb structural body such as cracks is generated is observed. In this case, the spalling strength is estimated by a safety temperature obtained as the highest temperature at which the honeycomb structural body shows no defect. The spalling strength using the electric furnace is defined by automotive standard M505-87 issued by Society of Automative Engineers of Japan, Inc. Moreover, as a fracture type due to the crack generation mentioned above, it is found that there are two types. That is, as shown in FIG. 1a, one crack type is a ring crack 4 in which the honeycomb structural body 1 is broken in the directions at a plane substantially perpendicular to an axial direction of passages 3 defined by partition walls 2. Further, as shown in FIG. 1b, the other crack type is an end crack 5 in which a crack is generated at one end surface or both end surfaces of the honeycomb structural body 1.
When the catalyst carrier including the honeycomb structural body is used in an automobile, it is better that the honeycomb structural body has a higher spalling strength mentioned above. This is because the higher spalling strength is an extremely significant characteristic in the case that the honeycomb structural body is exposed to a high temperature exhaust gas as compared with the conventional one or in the case that a temperature of the exhaust gas during a high speed driving becomes higher so as to improve fuel consumption, since it is necessary to arrange the catalyst carrier in the vicinity of the engine according to the severe exhaust gas regulation. Then, if the cracks mentioned above are generated during an actual use due to the low spalling strength, a decrease of exhaust gas purifying performance, a decrease of engine power in accordance with pressure loss increase, and a generation of noise from the exhaust system occur. From the view points mentioned above, even in the honeycomb structural body having an improved exhaust gas purifying performance, it is desired to obtain a honeycomb structural body having higher spalling strength.